Example embodiments relate to a gas sensor, and in particular, to composite metal oxide materials including polycrystalline nanofibers, microparticles, and nanoparticles, gas sensors using the same as a sensing material thereof, and methods of manufacturing the same.
Recently, a gas sensor with extremely high sensitivity is being extensively studied to early detect harmful gases or exactly detect minute amounts of volatile organic compound gases contained in an exhalation of a human being. Currently, resistance-type gas sensors are commercialized, and most of them include a sensing material composed of micro-sized metal oxide particles, each of which has a diameter ranging from several hundreds of nanometers (nm) to several micrometers (μm).
An operation of such a resistance-type gas sensor is performed based on a modulation in thickness of an electron depletion layer, which is caused by oxygen adsorbed on a surface of a sensing material (e.g., metal oxide semiconductor). The thickness of the electron depletion layer can be changed by reaction with an oxidizing or reducing gas. For example, and thus, if a sensing material is exposed to a target gas, the thickness of the electron depletion layer is increased to have an increased resistance or is decreased to have a decreased resistance. Here, in the case where the sensing material is formed of relatively large particles with an average particle size of micrometer order, it is difficult to increase a surface area of the electron depletion layer formed on surfaces of the particles.
In the case of using nanoparticles, the electron depletion layer is formed on each of the nanoparticles, and this makes it possible to greatly increase a total surface area of the electron depletion layer constituting the sensing material. Accordingly, the thickness of the electron depletion layer formed on the surfaces of the nanoparticles can be largely changed by the reaction with an external gas, and this leads to a large change in resistance of the electron depletion layer and an increase in sensitivity of the gas sensor. In fact, several papers reported that the nanoparticle-based gas sensor can have very high sensitivity. However, most of currently-commercialized gas sensors are manufactured using a sensing material composed of particles with an average particle size of micrometer or sub micrometer order.
Long lifetime reliability is another important property for the gas sensor, but in the case where the sensing material is only formed of nanoparticles, there is a difficulty in achieving the long lifetime reliability property of the gas sensor. As an example, if the gas sensor is repeatedly used for a long period of time, accuracy thereof may be degraded. As another example, in the case that the gas sensor is exposed for a long period of time to an environment of high temperature of about 300° C.-400° C., which is within its normal operating temperature range thereof, the nanoparticles may be reacted with each other to result in a change in shape of the sensing material (e.g., thermally expanded or cracked). Further, in the case where a sensor substrate or a sensor electrode is coated with dispersion solution containing nanoparticles, the nanoparticles may aggregate with each other and thereby have an increased particle size. In this case, the gas sensor may suffer from degraded sensing performance.
By contrast, in the case where a sensing material is formed of coarse (e.g., micrometer-sized) particles, it is possible to realize a gas sensor with extremely high thermal stability. However, since the sensing material composed of the micrometer-sized particles has a specific surface area that is several tens to several hundred times smaller than that of the case that the sensing material composed of the nanoparticles, it is hard to realize an extremely high sensitivity using the sensing material composed of only the microparticles. In particular, for the microparticle-based sensing material, it is impossible to detect minute amounts (e.g., several tens parts per billion (ppb)-one parts per million (ppm)) of volatile organic compound gases, which are contained in an exhalation of a human being. Meanwhile, the sensing material may be composed of a mixture of nanoparticles and microparticles. However, even in this case, the nanoparticles may be stuck on surfaces of the microparticles. Alternatively, the nanoparticles may participate in a nanoparticle-microparticle reaction, which may occur when a high temperature thermal treatment process is performed after coating a surface of a sensor electrode with the nanoparticles and microparticles. As a result, a density or compactness of the sensing material may be increased. Namely, the density or compactness of the sensing material may be too high for the gas sensor to detect a gas concentration of 1 ppm or lower.
Recently, one-dimensional nanowires or nanofibers are being studied for the gas sensor. Due to a large surface to volume ratio and a well-developed pore structure thereof, the use of nanowires or nanofibers allows for the gas sensor to have high sensitivity. However, in this case, since point-like contact regions between the nanowires or nanofibers are only included in a current path, a base resistance of the gas sensor in the air is very high. Accordingly, low resistivity materials (e.g., ZnO or SnO2) are usually used for the nanowires or nanofibers. If a high resistivity material is used, it is necessary to use an instrument capable of precisely measuring such high resistivity.